The pyrolysis of polyborodiphenylsiloxane has been studied at temperatures up to 1600 °C under an inert atmosphere. Above 1200 °C both a nano-scale β-SiC and an amorphous phase developed while at temperatures greater than 1400 °C significant mesoporosity started to appear, with surface areas and pore volumes reaching maximum values at around 1500 °C. The porosity decreased at higher pyrolysis temperatures and disappeared completely when the material was subsequently heated in an oxidising atmosphere at temperatures above 400 °C. The mesoporosity seems to be present in carbon rich areas which appeared when SiC started to crystallise. EELS showed that boron was present at all pyrolysis temperatures. Comparisons are made with the pyrolysis of polycarbosilane.